add JobLogger to Spark

add a new class named JobLogger
1.each SparkContext has one JobLogger, one folder is created for every JobLogger
2.JobLogger manages all history files of activeJobs running in that SparkContext,the file name is jobID
job history include:
1.additional information from outside
For example: queryplan from Shark
2.RDD Graph for each Job
The RDD graph is printed using a top-down approach
3.stage information and tasks' start/stop & shuffle information
gotten from TaskMetrics and DAGScheduler